Cells of the mesophyll make up the bulk of internal leaf tissue and are the major site of photosynthesis in a plant by virtue of containing large populations of chloroplast organelles. The differentiation of the mesophyll and its coordinated expansion is important to leaf function because light interception by chloroplasts and gas exchange in the internal airspaces of the leaf are crucial to optimise rates of photosynthesis. In dicotyledonous leaves there are two types of mesophyll cell; palisade mesophyll and spongy mesophyll. Palisade mesophyll cells are elongate and form a layer beneath the upper epidermis, whereas spongy mesophyll cells are internal to the lower epidermis. Mesophyll cells in monocotyledonous leaves are often highly lobed. All mesophyll cells contain large populations of chloroplasts, which enable the leaf to carry out photosynthetic carbon assimilation.

Key Concepts:

  • In dicotyledonous leaves there are two types of mesophyll cells, palisade mesophyll and spongy mesophyll.

  • Palisade mesophyll cells form a layer beneath the upper epidermis whereas spongy mesophyll cells are internal to the lower epidermis.

  • In monocotyledonous leaves, mesophyll cells are not differentiated into two types according to position in the leaf, although those cells circling the vascular tissue form a distinct cell‚Äźtype called the bundle sheath cell.

  • Mesophyll cells in monocotyledonous leaves are often highly lobed.

  • Mesophyll cells originate from the L2 and L3 layers of the shoot apical meristem.

  • A major role of leaf mesophyll cells is to contain large populations of chloroplasts, which carry out photosynthetic carbon assimilation facilitating plant growth.

Keywords: palisade mesophyll; spongy mesophyll; airspace; bundle sheath; chloroplast

Figure 1.

(a) Transverse section through a leaf of Arabidopsis thaliana. (b) Transverse section through a leaf of maize (Zea mays). PM, palisade mesophyll; SM, spongy mesophyll; BS, bundle sheath cells; V, vascular cells; and E, epidermis. Scale bar=50 μm. Reproduced from Hall LN and Langdale JA (1996) Molecular genetics of cellular differentiation in leaves. New Phytologist 132: 533–553.

Figure 2.

A palisade mesophyll isolated from a leaf of tomato (Solanum lycopersicon). The large population of green bodies is chloroplasts. Scale bar=10 μm.

Figure 3.

Two mesophyll cells isolated from a young leaf of wheat (Triticum aestivum). Note the elongated axis and the lobing of each cell. Such mesophyll cells in wheat vary greatly in their length and degree of lobing. Large populations of green chloroplasts are visible in each cell. Scale bar=10 μm.

Figure 4.

(a) Three isolated mesophyll cells from a leaf of Arabidopsis thaliana. P – palisade mesophyll cell in addition to S – spongy palisade cell. (b) Isolated mesophyll cell from a leaf of rice (Oryza sativa). Note its small size and its lobed structure. These cells contain small numbers of green chloroplasts. Scale bar=10 μm. Image by Ian Smillie.



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Further Reading

Fabrini M, Innocenti ED, Cionini G, Pugliesi C and Guidi L (2010) Mesophyll cell defective1, a mutation that disrupts leaf mesophyll differentiation in sunflower. Photosynthetica 48: 135–142.

Micol JL (2009) Leaf development: time to turn over a new leaf. Current Opinion in Plant Biology 12: 9–16.

Steeves TA and Sussex IM (1989) Patterns in Plant Development, 2nd edn, chap. 9. Cambridge: Cambridge University Press.

Tilney‐Bassett RAE (1986) Plant Chimeras. London: Edward Arnold.

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Pyke, Kevin(Jun 2012) Mesophyll. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0002081.pub2]